1. Field of the Invention
The present invention relates to mass spectrometry. In particular, the present invention relates to noise reduction in negativeion quadrupole mass spectrometry.
2. Discussion of Background
A mass spectrometer is an analytical instrument used to determine the composition of a gas sample. The sample is ionized and the ions formed into a beam that is accelerated through a mass filter (magnetic or resonant) to separate the ions according to the ratio of their electric charge to their mass (e/m ratio). The numbers of ions of each e/m ratio is counted and recorded as a mass spectrum which will typically have peaks characteristic of the ionized species found in the sample. Gas samples or samples of volatile substances can be analyzed directly; nonvolatile solid or liquid samples must be converted to a vapor by an electric arc, laser heating, or other means.
Mass spectrometers can accurately measure even very small concentrations of ions, and thus are useful tools for chemical analysis. Specialized techniques based on mass spectrometry include secondary ion mass spectrometry (SIMS), Auger spectroscopy, electron scattering for chemical analysis (ESCA), ion probe mass spectrometry, and inductively coupled plasma (ICP) mass spectrometry. Mass spectrometers can also be used as leak detectors and residual gas analyzers.
A mass spectrometer includes three basic components: an ion source, which produces a beam of ionized particles from a sample; a means for separating different ion types in the beam by e/m ratio; and a detector, which measures and records the intensity of each of the types.
Many different ion sources are available. For example, gas samples may be ionized by an electron beam in an ionization chamber; solid or liquid samples may be vaporized by a laser beam, resulting in ejection of neutral atoms and charged particles that form a plasma; actinide samples may be obtained by using thermal ionization filaments. Recent developments include ion production by inductively coupled plasma torches, fast atom bombardment of liquid surfaces, and ionizing the surface molecules of a liquid sample with weak laser light. In a typical mass spectrometer, ions exiting the source are formed into a beam and accelerated by an eletric potential V before entering the magnetic field where they are separated according to their mass and charge. The radius of curvature of the path of an ion with mass m and charge e in a magnetic field H is EQU R=(1/H)(2Vm/e).sup.1/2.
For fixed V and H, only those ions with a particular value of m/e will reach the detector. Most of the ions produced by the source have a charge of 1 electron unit, so ions of any desired mass can be collected simply by adjusting V or H. The mass spectrum is obtained by varying V or H and recording the resulting peaks. The magnetic field is provided by an electromagnet or high field strength permanent magnet.
A quadrupole mass spectrometer (QMS) can separate ions according to their m/e ratio without using a heavy permanent magnet or electromagnet. Instead, a quadrupole mass filter--an array of four straight, parallel metallic rods--is positioned so that the ion beam passes down the center of the array. Such a system is illustrated schematically in FIG. 1. QMS system 10 includes ion source 12, quadrupole mass filter 14, and ion collector/recorder 16. Quadrupole 14 has entrance plate 20 with aperture 22, and exit plate 24 with aperture 26. Four parallel metal rods 28, 30, 32, 34 extend between plates 20 and 24 as shown. Rods 28, 30, 32, and 34 have circular cross-sections. Opposing rods 28 and 30 are electrically connected by connector 36; opposing rods 32 and 34 are similarly connected by connector 38. Paired rods 28, 30 and 32, 34 are connected to opposite poles of variable DC source 40, and simultaneously to radio frequency source 42 in parallel with capacitor 44. The amplitude and frequency of RF source 42 are variable.
Incident particle beam 50 from ion source 12 enters quadrupole 14 through aperture 22. The forward motion of the ions in beam 50 is not affected by the DC field from source 40 or the RF field from source 42, since neither field has a component parallel to rods 28, 30, 32, 34. Only the lateral motion of the ions is affected by the fields.
FIG. 2 shows a cross-sectional view of quadrupole 14 through plane A of FIG. 1. Rods 28, 30, 32, 34 form square array 70 about origin 72. Each rod is a distance r (indicated by reference character 74) from origin 70. To a good approximation, the potential .PHI.as a function of time t at a point (x,y) in plane A is EQU .PHI.(t)=(V.sub.dc +V.sub.0 cos.omega.t)(x.sup.2 -y.sup.2)/r.sup.2
where V.sub.dc is the applied DC potential of source 40. V.sub.0 and .omega. are the amplitude and frequency, respectively, of the RF potential generated by source 42. The x and y components of the lateral force F on an ion with charge e moving between the rods are EQU F.sub.x =-e(d.PHI./dx)=-(e/r.sup.2)(V.sub.dc +V.sub.0 cos.omega.t)2x EQU F.sub.y =-e(d.PHI./dy)=-(e/r.sup.2)(V.sub.dc +V.sub.0 cos.omega.t)2y,
so the equations of motion are EQU d.sup.2 x/dt.sup.2 +(2/r.sup.2)(e/m)(V.sub.dc +V.sub.0 cos.omega.t)x=0 EQU d.sup.2 y/dt.sup.2 -(2/r.sup.2)(e/m)(V.sub.dc +V.sub.0 cos.omega.t)y=0
The lateral motion of the ion in plane A is therefore proportional to e/m, with a periodic component of frequency .omega.. For fixed V.sub.dc and V.sub.0, there is a narrow frequency range within which this motion is confined to the space between rods 28, 30, 32, 34. Resonant ions having a frequency within this range pass through array 70 without colliding with one of the rods. For fixed V.sub.dc, V.sub.0, and .omega., only ions with a specific e/m ratio pass through array 70. Lighter or heavier ions drift outwards and collide with one of the rods.
Resonant ions 52 pass through array 70, exiting through aperture 26 as beam 56 and passing to ion collector/recorder 16 (FIG. 1). Ion collector 16 includes a means for collecting ions such as a Faraday cage or electron multiplier, and a means for recording a mass spectrum. Nonresonant ions 54 do not reach ion collector 16. It will be understood that QMS system 10 may have different arrangements of ion source 12, quadrupole 14, and ion collector 16. For example, beam 56 may be deflected to an off-axis detector, or accelerated after passing through aperture 26 by a post-acceleration plate (not shown) located between aperture 26 and detector 16.
The ions in incident beam 50 are selected according to their e/m ratio by varying the RF frequency .omega. while maintaining the ratio V.sub.dc /V.sub.0 constant. Lighter ions (such as H, He) pass through array 70 at high frequencies, and heavier ions (such as Pb, the actinides, heavy organics) at lower frequencies. The mass spectrum of exiting beam 56 is obtained by collecting particles of different e/m ratios at ion collector 16 as the frequency is varied. Signal-to-noise ratios in the parts per billion (ppb) range can be obtained, so QMS is a sensitive technique for chemical analysis.
A common problem in QMS systems is the presence of undesired particles in the ion beam. Since ion collector 16 records the presence of a charged particle, not its sign, QMS system 10 is most sensitive when all the ions in beam 56 have the same charge. Whether QMS 10 is operated in positive or negative ion mode, the presence of oppositely-charged particles contributes to the background noise level and thereby reduces the signal-to-noise ratio and sensitivity of the system. The positive ionization mode is most commonly used. However, the negative ionization mode is theoretically superior for electro negative elements such as the halogens.
All like-charged ions can be removed from a beam by simple electrostatic techniques well known in the art, leaving a beam having only negative or only positive ions. A number of other techniques use electric or magnetic fields to further separate out the undesired components of a particle beam. Electric fields are used to select charged particles having a desired range of kinetic energies (Fite, U.S. Pat. No. 4,146,787; Wardly, U.S. Pat. No. 3,679,896); to reduce the background in ion probe mass spectrometry by deflecting low energy sputtered ions into the entrance aperture of the mass analyzer (Maul et al., U.S. Pat. No. 3,922,544; to deflect an ion beam to either a monitor or an ion collector (Nakajima, U.S. Pat. No. 3,764,803); and to remove electrons from a plasma by passage through successive electric fields of increasing amplitudes (Eloy, U.S. Pat. No. 3,644,731). Electrostatic fields are used to deflect the carrier gas ions from impinging on the beam monitor electrode in combined gas chromatography-mass spectrometry (McCormick, U.S. Pat. No. 3,641,339). Liebl (U.S. Pat. No. 3,617,739) shows an ion microprobe apparatus in which test objects can be selectively irradiated by ion beams or electron beams focused by magnetic lenses.
However, techniques such as those described above cannot readily separate electrons from other negatively-charged particles with similar kinetic energies. It is therefore especially difficult to achieve good signal-to-noise ratios when a QMS system is operated in the negative ion mode, since electrons are always produced when a negative-ion beam is formed. Furthermore, secondary ions and electrons may be generated within the system, such as when an ion strikes one of rods 28, 30, 32, 34, or when the ion beam hits a deflector, or an acceleration or post-acceleration plate (if present). These noncollimated, low-energy electrons have an essentially random energy distribution, with a maximum energy typically no more than about 70 eV of these electrons and secondary ion have sufficient energy to pass through quadrupole 14 and enter ion collector 16, adding to the background in the recorded mass spectrum. Due to their small mass (1/1837 amu), electrons with energies as low as 1 eV may pass through a quadrupole 14 without being deflected. While this problem is seen with all negative-ion beams, it is especially evident when the source of negative ions is an inductively coupled plasma (ICP) torch. The resulting high noise levels severely limit the detection limits of a QMS system.